1. Field of the Invention
This invention relates generally to down hole tools for use in oil and gas wells, and more particularly to down hole tools having drillable materials and metallic slips.
2. Related Art
Down hole tools, such as well packers, bridge plugs, fracture (“frac”) plugs, cement retainers, and the like, are commonly used in oil or gas wells for fluid control in both completion and production efficiency applications. For example, such down hole tools are often placed in the bore of a well to form a seal between the well tubing and casing in order to isolate one or more vertical portions of the well. A tool can also be placed inside the casing to isolate one elevation from another during formation fracturing and treatment operations.
Down hole tools often have central mandrel with lower slip elements adjacent a lower slip wedge and upper slip elements adjacent an upper slip wedge. The slip elements are often made of a cast iron material, composite material or the like, so as to facilitate drill out when removal of the down hole tool is desired. Additionally, a compressible packer is disposed between the upper and lower slip elements. The compressible packer is often made of an elastomeric material such as rubber so that the compressible packer can conform to the shape of the surrounding well bore and down hole tool in order to form a seal between the well bore wall or casing and the central mandrel.
In use, the down hole tool is positioned in the well bore at a desired depth and an axial force is applied to the upper and lower slip segments such that the upper and lower slip segments are moved closer together along the longitudinal axis of the central mandrel so as to compress the compressible packer. As the compressible packer is compressed, the packer bulges radially outward to form a seal between the central mandrel and the well bore wall or casing. Additionally, the upper and lower wedges are forced under the upper and lower slip elements, respectively, to force the slip elements radially outward away from the mandrel toward the well bore wall or casing in order to set the tool in the well bore by engaging the well bore wall or casing.
Because down hole tools are used in a wide range of well bore environments, they must be able to withstand extremes of high temperature and pressure as well as corrosive fluids, such as acid or brine solutions, superheated water, steam, and other natural formation fluids, as well as fluids used in oil or gas well operations. During normal well completion operations, the down hole tools must be removed to allow the installation of tubing to the bottom of the well to begin the recovery of oil and gas. In order to facilitate removal of these tools, the components are usually made of easily drillable materials, such as cast iron, fibrous composite materials, and the like.
Unfortunately, the down hole tools described above have some problems For example, the slip elements are often made of a cast iron ring with stress risers spaced about the ring. The stress risers are configured to fracture the ring into separable slip elements when the slip wedges apply radial forces on the cast iron ring. Unfortunately, the rings sometimes do not fracture along the stress risers, or the stress risers do not fracture uniformly so that the separable slip elements are not evenly formed. When this happens one of the separable slip elements may be larger than another so that when the slip elements engage the well bore wall or casing an uneven loading is applied around the central mandrel. This uneven loading can result in movement of the down hole tool over time as it is used in the well bore and which results in an loss of seal or damage to other well components.
Another problem of the down hole tools described above is that the cast iron rings that separate into the slip segments often fracture into the separable segments at nearly the same time. This can result in setting of the tool in the well bore before the compressible packer is sufficiently compressed to form an optimal seal between the central mandrel and the well bore wall or case.
Still another problem of the down hole tools described above is that the compressible packer is often exposed to a wide range of temperatures. Sometimes the temperatures can soften or melt the polymer of the compressible packer such that the packer material can flow under pressure around the slip wedge and through the gaps between the separated slip elements such that the integrity of the seal can be compromised. Alternately, the packer material can flow into the gap between the conical wedge outer diameter and the casing inside diameter.